[souffle]

If you cannot afford a vet then you need to take her back and insist she is seen by a vet then returned to you. Don't let them replace her but insist she is checked once the vet has seen her and treated her. It could be a number of things ranging from a bite wound to a skin infection so it really does need checked out. I wouldn't touch the area until it has been checked out.
Start setting up a vet fund now by putting some money aside every week and not touching it till needed. Even just the cost of a coffee every week soon mounts up for future emergencies.